Low invasion coring fluid



United States Patent 3,314,489 LOW INVASION CORING FLUID Raymond A. Humphrey, Tulsa, Okla., assignor, by mesne assignments, to Esso Production Research Company, a corporation of Delaware No Drawing. Filed Oct. 30, 1964, Ser. No. 407,906 7 Claims. (Cl. 17559) The present invention is generally concerned with the sampling of underground formations. The invention particularly relates to a method of rotary core drilling to obtain core samples of essentially unaltered fluid content. Specifically, the invention is directed to the formulation of a novel oil base core drilling fluid containing Gilsonite.

The stock tank barrel volumetric petroleum content of a porous underground reservoir is normally calculated in terms of the reservoir volume, the formation volume factor, the average reservoir porosity and the average oil saturation within the porosity. In reservoirs wherein a number of development wells have been drilled, the reservoir volume and the formation volume factor can usually be determined with reasonable accuracy by comparing structural maps and measuring the gravity, tem perature, pressure and gas content of the oil under reservoir conditions. The average porosity of the reservoir is generally determined by analyzing cores recovered from a number of development wells.

Experience has demonstrated, however, that volumetric oil content in the cores, as determined from core analysis, is usually not an accurate indication of the actual quantity of oil present in a reservoir. Reliable values for volumetric oil content, together with the other data, are extremely desirable to forecast the productive life of oil and gas reservoirs, to select the primary recovery techniques most suitable for particular reservoirs, and to assess the susceptibility of such reservoirs to later secondary and tertiary recovery processes.

Accordingly, it is a principal object of the present invention to obtain samples from a reservoir, the analysis of which will provide reliable values for volumetric oil content. It is a further object of the invention to provide a novel drilling fluid for circulation during a coring operation.

Conventional core drilling systems utilize an annular bit and core barrel which are rotated from the earths surface by means of a rotary drilling string. A coring fluid is circulated downwardly through passages in the drill string, barrel and bit in order to maintain pressure on the formation and thus prevent the escape of fluids contained therein. Cuttings produced by the bit are entrained in the coring fluid and returned to the surface through the annulus surrounding the drill string. As the bit cuts away the formation the central core which remains is encased in the barrel. The core barrel is provided with means for breaking off the core when the barrel is filled. Pressure core barrels which can be sealed against changes in pressure are also used. After the core is cut, the drill string is withdrawn from the borehole and the core thereby recovered.

It has been shown that the pressure maintained at the bottom of the borehole during a coring operation has a critical eifect on the fluid content of the cores subsequently recovered. If the pressure is less than the formation pressure, fluids contained in the formation will tend to flow out of the core into the borehole until equilibrium is established. If, on the other hand, the bottom hole pressure exceeds the formation pressure, the coring fluids will tend to flow into the interstices of the formation thereby displacing any oil, gas or water contained therein. In either case, the result is a change of the fluid content of a core, whereby measurements of the amount of fluids therein will not accurately reflect the original fluid content of the cored formation. Since this change in fluid content occurs as the core is cut, the use of a pressure core barrel cannot prevent it.

Several methods for avoiding the difliculty outlined above have been proposed in the past. The most obvious of these involves conducting coring operations without any pressure differential between the core and the formation. This is impractical, if not impossible, because the formation pressure cannot be conveniently measured during core drilling, and moreover, because the fluid pressure cannot be controlled with sufficient accuracy. The circulation during rotary core drilling of a drilling fluid which resists invasion under pressures considerably in excess of the formation pressure has been disclosed, but efforts to develop an ideal fluid have not been entirely successful. Systems which include the use of radioactive tracers to permit a determination of the extent to which core invasion has occurred, and systems for freezing the core in situ, have been proposed but have not been found to be generally elfective.

Heretofore the most promising of low invasion coring fluids which have been developed are polymeric elastomer latices. These materials are aqueous dispersions of oil resistant polymeric elastomers, including natural and synthetic rubber latices, both virgin and reclaimed. A substantial degree of success has been achieved with these fluids, particularly in the coring of sandstone formations. However in the coring of limestone formations, it has been found that these fluids do not satisfactorily prevent filtrate invasion.

Oil base muds containing Gilsonite as a fluid loss additive have been proposed in the past for ordinary drilling operations. However, such prior fluids do not sufliciently resist filtration loss when employed in rotary core-drilling to obtain samples having a fluid content truly representative of the surrounding formation. Actually, many proposed core drilling fluids have been shown to result in the flushing of more than 20% of the pore volume of recovered core samples, by filtrate invasion, after having produced negligible filtrate when subjected to the standard API test (30 min.) for filtration loss. It is apparent therefore, that the static exposure of a porous surface to a drilling fluid is not a satisfactory indication of the behavior of the fluid in the dynamic environment of a rotary core drilling operation. Filtrate invasion during rotary core drilling is greatly increased by a repeated exposure of fresh rook surface to a flowing stream of drilling fluid, caused by a continuous or repeated removal of the initial film or filter cake which is deposited as the fluid first contacts a given rock surface.

In accordance with the present invention, it has been found that the formulation of a satisfactory oil base coring fluid containing Gilsonite requires the selection of an oil base having an aniline point of at least F., preferably at least F., and having a viscosity within the range of 10 to 100 centipoises at 100 F. It is also essential that the fluid be composed of 20 to 45 percent -Gilsonite by weight, and preferably 35 to 45 percent by weight.

Pulverized Gilsonite is commercially available in three grades, designated Selects, VB and Sparkling Black by the American Gilsonite Company of Salt Lake City,

Utah. The commercial powder is too coarse to provide a satisfactory fluid for the purposes of the present invention. The particle size of the Gilsonite may be reduced to colloidal dimensions by passing an oil suspension of the commercial powder through a colloid mill. It is also possible to reduce the particle size of the commercial Gilsonite powder after it has been dispersed in an oil base, by heating the suspension to a temperature of at least 120 F. for a period of at least 30 minutes. In some reservoirs, where the average temperature is above 120 F., the heating step may be carried out by simply circulating the fluid downhole, prior to coring, for a time suflicient to heat the fluid.

Some formulations require heating for several hours. When a large volume of fluid is being prepared, the most suitable heating time for a given formulation can be de termined by periodically sampling the mixture, cooling the sample to ambient temperature, and determining its viscosity. Since the mixture steadily becomes more viscous upon continued heating, it is essential that it be cooled before it becomes too viscous for handling. The heating time may be reduced by increasing the temperature; however, an excessive increase in temperature is undesirable, since large volumes of fluid cannot be quickly cooled. For example, when the Selects grade of Gilsonite is employed, excessive aging at elevated temperatures may be virtually unavoidable, once a temperature in excess of 140 F. has been reached. Oil suspensions of the VB and Sparkling Black grades of Gilsonite require a more severe heat aging than the Selects.

The mixing oils usually employed in the formulation of commercial oi-l base drilling fluids are diesel oil and crude oil. The aniline point of a diesel oil usually falls within the range of about 130 to 160 F., while the aniline point of a crude oil is generally somewhat lower. The viscosity of these mixing oils at 100 F. falls within the range of 1 to 5 centipoises. These oils are unsuitable for use in the formulation of the coring fluids of the present invention.

Particularly suitable mixing oils are the heavy petroleum distillates obtained from paraffinic or intermediate crudes, including primarily the gas-oil distillates and lubricatingoil stocks, boiling in the 600 to 900 F. range. Petroleum white oils, for example, fall in this category, and have been found suitable for use in the present invention.

The drilling fluid may be tagged with a radioactive tracer in order to permit the subsequent core analysis to distinguish between the oil initially contained Within the pore space of the rock, and the oil present due to drilling fluid invasion.

For convenience in handling, it is preferable to incorporate in the fluid a small quantity of suspending or gelling agent to provide increased stability against settling of the dispersed Gilsonite on standing. An alkaline earth soap is particularly suitable for this purpose.

Core drilling operations utilizing the coring fluid of the invention may be carried out with conventional apparatus familiar to those skilled in the art. A variety of commercially available core bits and core barrels may be used. The coring fluid is circulated down the drill string through channels in the core barrel and bit from which it emerges adjacent the cutting surfaces of the bit. The fluid contacts the core surfaces as the surrounding rock is cut away and continuously forms a film on the surface which is impermeable to oil, gas and water thus preventing any significant alteration of the original fluid saturation in the core. Once a core of sufficient length is cut, it is separated from the formation by conventional means, and is lifted to the surface inside the core barrel.

The following formulation was prepared as an example of the drilling fluids of the present invention:

Mixing oil "gallons" 34 Pulverized Gilsonite Selects lbs 100 Lime lbs 3.5 Tall oil gallon /2 Weight on bit, lbs 6000 Penetration rate, ft./hr. 2-2.5 Rotary speed, r.p.m 60 Circulation rate, gals/min. 70 Pressure differential, p.s.i 500 Invasion of core by drilling fluid, 3%.

The following filtration data illustrate the necessity of selecting a mixing oil having an aniline point of at least 175 F. Each fluid contained 25 grams of Gilsonite Selects dispersed in 75 ml. of a mixing oil having the indicated aniline point. The fluids were subjected to the API standard filtration test for the indicated times.

Aniline point, F.. 155 166 173 18G Milliliters filtrate After:

1.0 minute 3 minutes 30 minutes 60 minutes 200 minutes 245 minutes What is claimed is:

1. An oil-base coring fluid comprising from 20 to 45 weight percent Gilsonite dispersed in a mineral oil having an aniline point of at least 175 F., and a viscosity of 10 to centipoises at 100 F.

2. An oil base coring fluid comprising from 20 to 45 weight persent Gilsonite coll-oidally dispersed in a petroleum distillate oil having an aniline point of at least 175 F., and a viscosity of 10 to 100 centipoises at 100 F.

3. An oil base coring fluid prepared by dispersing from 20 to 45 weight percent pulverized Gilsonite in a hydrocarbon oil having an aniline point of at least 175 F., and a viscosity of 10 to 100 centipoises at 100 F., and then heating the mixture to at least F. for at least 30 minutes.

4. In the recovery of core samples from a borehole in the earth by rotary core drilling, wherein a non-invading fluid is circulated down the hole, the improvement which comprises circulating as said fluid a composition comprising a dispersion of '20 to 45 weight percent Gilsonite in a petroleum fracton having an aniline point of at least F. and a viscosity of 10 to 100 centipoises at 100 F.

5. A process for the recovery of core samples from a borehole in the earth which comprises advancing a rotary core bit into the earth-at the bottom of the borehole while circulating downhole a suspension of 20 to 45 weight percent finely divided Gilsonite in mineral oil, whereby the samples recovered are sealed substantially as cut, thereby capturing the original fluid saturations contained therein, said oil having an aniline point of at least 175 F., and a viscosity of 10 to 100 centipoises at 100 F.

6. A process as defined by claim 5 wherein said suspension comprises at least 35 weight percent Gilsonite.

7. A process as defined by claim 5 wherein the oil of said Gilsonite suspension has an aniline point of at least about F.

(References on following page) 5 6 References Cited by the Examiner OTHER REFERENCES UNITED STATES PATENTS Rogers, Walter F., Composition and Properties of Oil 2 1 968 4/1943 Miller 5 5 V611 Drilling Fluids, pp. 401-404, 1948. 2,445,494 7/1948 Redmond 17559 X i 2,773,670 12/1956 Miner 175 72 5 CHARLES E. O CONNELL, Pnmaiy Examzner. 2,812,161 11/1957 Mayhew 175 72 X I. A. CALVERT, Assistant Examiner.

3,036,633 5/1962 Mayhew 166-31 

1. AN OIL-BASE CORING FLUID COMPRISING FROM 20 TO 45 WEIGHT PERCENT GILSONITE DISPERSED IN A MINERAL OIL HAVING AN ANILINE POINT OF AT LEAST 175*F., AND A VISCOSITY OF 10 TO 100 CENTIPOISES AT 100*F.
 4. IN THE RECOVERY OF CORE SAMPLES FROM A BOREHOLE IN THE EARTH BY ROTARY CORE DRILLING, WHEREIN A NON-INVADING FLUID IS CIRCULATED DOWN THE HOLE, THE IMPROVEMENT WHICH COMPRISES CIRCULATING AS SAID FLUID A C OMPOSITION COMPRISING A DISPERSION OF 20 TO 45 WEIGHT PERCENT GILSONITE IN A PETROLEUM FRACTION HAVING AN ANILINE POINT OF AT LEAST 175*F. AND A VISCOSITY OF 10 TO 100 CENTIPOSES AT 100*F. 